1. Field of the Invention
The present invention is related to a coding mode determination system for moving pictures, and particularly, to a coding mode determination system for performing a mode determination as to whether the coding is to be performed between frames or in a frame, which is suitable for providing a highly efficient coding by adaptively selecting an inter-frame predictive coding mode and an intra-frame coding mode.
2. Description of the Related Art
Conventionally, in the inter-frame motion compensated predictive compression coding represented by the ITU-T recommendation H.262 (MPEG-2), a picture is divided into many rectangular blocks of, for instance, 16 pels.times.16 lines, called macro-blocks, and motions in the picture are estimated for each block to provide a motion compensated prediction.
Further, to actually encode a motion compensated predictive difference or encode the pel value itself in a frame rather than predictive difference is determined according to a certain criterion from the viewpoint of coding efficiency.
For instance, the intra-frame coding is selected if a dispersion of the intra-frame pel values in a macro-blocks is smaller than a dispersion of the predictive difference in the macro-blocks, otherwise the inter-frame coding is selected. That is, the dispersion of the pel values of a macro-block X of 16 pels.times.16 lines in a frame (hereinafter referred to as intra-frame picture signal X) is obtained in a first dispersion calculation unit 41, and the dispersion of the motion compensated predictive difference signal Y of the macro-block (hereinafter referred to as predictive difference signal Y) is obtained in a second dispersion calculation unit 42 shown in FIG. 5. The dispersion values obtained in the first and second dispersion calculation units 41 and 42 are compared in a comparator unit 43, and if the dispersion value of the intra-frame picture signal X is smaller than the dispersion value of the predictive difference signal Y, a signal for selecting the intra-frame coding, or an intra-selection signal Z, is outputted from the comparator unit 43, and conversely, if the dispersion value of the predictive difference signal Y is smaller than the dispersion value of X, a signal for selecting the inter-frame coding, or an inter-selection signal Z, is outputted from the comparator unit 43.
However, in the standard coding method represented by the MPEG-2 or the like, a macro-block is further divided into four sub-blocks (size: 8 pels.times.8 lines), and each of the four sub-blocks is encoded by an orthogonal transformation coding such as DCT.
The amount of generated encoded information in coding is important for encoding efficiency, but the conventional dispersion compare in a macro-block does not always correspond to whether such amount is large or small, and thus there is a problem that it is not always an optimum determination method in the sense of mode determination that the coding efficiency is maximized.